Re: [peterz-queue:sched/core 13/19] kernel/sched/fair.c:892:34: warning: variable 'stats' set but not used

On Sat, Sep 11, 2021 at 04:30:41PM -0500, Segher Boessenkool wrote:

Hi!

On Sat, Sep 11, 2021 at 02:20:49PM +0200, Peter Zijlstra wrote:

quoted

On Sat, Sep 11, 2021 at 02:21:26AM +0800, kernel test robot wrote:

quoted

compiler: m68k-linux-gcc (GCC) 11.2.0

quoted

   kernel/sched/fair.c: In function 'update_curr':
   kernel/sched/fair.c:860:42: warning: unused variable 'stats' [-Wunused-variable]
     860 |                 struct sched_statistics *stats = __schedstats_from_se(curr);
         |                                          ^~~~~

OK, compiler guys, this code reads like:

#define schedstats_enabled()	(0)
#define __schedstat_set(x, y)	do { } while (0)


	if (schedstats_enabled()) {
		struct sched_statistics *stats = __schedstats_from_se(curr);

		__schedstat_set(stats->exec_max,
				max(delta_exec, stats->exec_max));
	}

So yes, we initialize a variable that then isn't used, but the whole
bloody thing is inside if (0) which will not ever get ran *anyway*.

This is a crap warning if ever I saw one...

Yes, we really should warn "do not use a preprocessor macro if what you
want is a function"?  The variable really *is* unused, with this macro.

Why would I want to write a bunch of one-off functions and preprocessor
guard them? That's going to be a mess.

If we would remove dead code before warning about unused variables
there would be many *more* false positives, fwiw.

Not if you also remove any variables declared in dead code and all
variables only used in the dead code.